Re: LEFT JOINs not optimized away when not needed

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Jul 8, 2014 at 5:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Moshe Jacobson <moshe(at)neadwerx(dot)com> writes:
>>> On Tue, Jul 8, 2014 at 4:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>>> No. There is nothing about GROUP BY in the join removal logic.

>>> OK. Thank you. Is this something that would make sense to add?

>> Dunno. I don't recall any previous requests for such a thing, so I'd not
>> be inclined to add it unless it can be done very cheaply (in terms of both
>> code and runtime).

> I thought this was exactly what was being discussed on the "Allowing
> join removals for more join types" thread.

No, this is a completely different thing. The idea is that if the query
is grouping on outer-relation columns, you don't need to care if the inner
relation is unique or not, because it doesn't matter if there are multiple
matches.

regards, tom lane